Understanding intersex

The intersex flag (learn more).
Of course, the idea that groups
need such flags is contentious.

We tend to think of human beings as dividing into either the male sex or the female sex. But biology is not that neat. The US historian Alice D. Dreger put it succinctly:

Humans come in a wonderful array of types: many sizes, many abilities, many features, and many approaches to experiencing and organising the world. Human variation is truly extraordinary. Amid all that variation, we can say with some confidence that most human beings develop one of two common sets of a particular group of organs. That is, most people possess either what is now labelled “female” or “male” sexual anatomy... Yet some people – more than is generally assumed – are born with an anatomical conformation different from “standard” male or female bodies. Their unusual anatomies can result in confusion and disagreement about whether they should be considered female or male or something else.[1]

On the normal (i.e. most commonly occurring) model, a person’s sex is determined by having either XX (female) or XY (male) chromosomes, determined at the moment of conception. This leads to possession of one or the other set of characteristic internal and external sexual / reproductive organs. The table below sets out the typical patterns:

	Male	Female
Chromosomes:	46,XY	46,XX
Phenotypic pathway:	Wolffian	Müllerian
External sex organs:	Penis, scrotum, epididymis, and testes	Mons pubis, pudendal cleft, labia majora and minora, vulva, Bartholin’s gland, and the clitoris
Internal sex organs:	Accessory organs: vas deferens, seminal vesicles, prostate gland, bulbourethral glands	Vagina, uterus, fallopian tubes, cervix, and ovary
Hormones:	High testosterone, low oestrogen	High oestrogen, low testosterone
Gamete:	Sperm	Egg

(For anatomical terms that are unfamiliar you can follow these links for male and female.)

Sex is a naturally occurring phenomenon which we humans abstract into the binary concepts of male and female. The vast majority of human beings fits completely unambiguously into one of these two categories. 

However, in practice some people are born with characteristics that do not line up in this neat way. Other arrangements are possible, and unusual anatomy (structure of body parts) and/or physiology (the functions and relations of those parts) can create confusion over how the person should be categorised. 

Such people are often called intersex. This word draws on the Latin inter or ‘between’ to mean ‘between sexes’, and has only been in use in its modern sense for about one hundred years. The English word ‘intersex’ has been around since the late 18th century, but prior to the 20th century it was uncommon and referred to relations between men and women. It was only in 1917 that Richard Goldschmidt introduced the term ‘intersexuality’ [2], in an academic paper on endocrinology interested in how the existence of ‘an intergrade between the two sexes’ in moths can cast light on how sex differentiation works. 

Today ‘intersex’ (in the broadest definition of the word) is usually used as an umbrella term for a wide range of 40+ birth conditions that pertain to sexual characteristics. Intersex people have chromosomal, hormonal or genital variations that diverge from the (statistical) norm. They may have, for example:

• ambiguous genitalia
• one sex’s external organs and the other’s internal organs
• aneuploidies i.e. chromosomes other than 46,XY or 46,XX 
• e.g. XXY or Klinefelter Syndrome (only affects men) in which there is an extra X chromosome
• e.g. XO or Turner Syndrome (only affects women) in which one of the X chromosomes is missing or damaged
• different chromosome patterns in different cells
• untypical hormone levels (a male with high oestrogen, a female with high testosterone)

There are many conditions that might be considered intersex – there is a comprehensive list on Wikipedia – and there is a stricter definition that will be discussed below. Some are obvious at birth, others don’t get noticed until puberty, and it is possible to be intersex your whole life without realising it (e.g. untypical internal organs you never know are there). They are all empirically observable conditions based on physiology, diagnosed medically: being intersex is a material reality about your body, not an identity, feeling or belief. 

Since humans (and life forms in general) are diverse and complicated, the existence of atypical cases is not surprising and should in fact be expected. Even unambiguous males and females show a great deal of variation: there are flat-chested women and big-breasted men, hairy women and relatively hairless men, tall women and short men. So asking who is intersex and who isn’t raises questions about our sex categories and how we think about what is ‘normal’. 

‘Hermaphrodites’

For centuries, intersex people were referred to as ‘hermaphrodites’. This term comes from Hermaphroditus, who in Greek mythology was the very handsome son of Hermes (the god of travellers and the messenger of the gods) and Aphrodite (the goddess of love and beauty) and is named after them both. The Roman poet Ovid in his Metamorphoses describes the rather horrible story: the nymph Salmacis falls in love with Hermaphroditus, rapes him, and prays for them to be combined into one (without the boy’s consent), a prayer which the gods answer. 

The nymph and the boy were no longer two, but a single form, possessed of a dual nature, which could not be called male or female, but seemed to be at once both and neither.[3]

This is the only full account of the myth. Hermaphroditus became a symbol of androgyny [4] wrapped up with sexual fantasy, incarnated in the Greco-Roman sculptural trope of the ‘sleeping Hermaphroditus’: figures who look female when viewed from behind but have male genitalia on the front. The Greek historian Diodorus Siculus observed a negative twist to the story:

There are some who declare that such creatures of two sexes are monstrosities.[5] 

In biology, a hermaphrodite is defined (according to the OED) as:

A person or animal having both male and female sex organs or other sexual characteristics, either abnormally or (in the case of some organisms) as the natural condition.

In some species (such as snails, slugs and earthworms) this is normal, but hermaphroditism, in the sense of fully male and female sexual organs and functions occurring in the same individual, does not exist in humans. There has never been a single observed case in Homo sapiens. Intersex people therefore are not hermaphrodites. The closest that real human beings come to hermaphroditism is ovotesticular disorder, when someone is born with both testicular and ovarian tissue – which affects just one person in 100,000 and still does not make someone hermaphroditic. 

In a 2005 paper [6], five authors argued that ‘hermaphrodite’ and the derivative terms in use at the time (e.g. ‘pseudohermaphrodite’) were ‘scientifically specious and clinically problematic’, focussed exclusively on the gonads and failed to reflect advances in the scientific understanding of sex that allowed more accurate and useful terms. Their suggestion was to refer instead to Disorders of Sexual Development, often abbreviated to DSDs. Some people use ‘Differences’ rather than ‘Disorders’ as they find it less negative, or prefer Variations of Sexual Development or VSDs for the same reason. DSDs is now the most commonly preferred term, as ‘intersex’ gets defined in different ways and is not useful from a medical/clinical perspective. The term ‘intersex’ too can be seen as inaccurate, as it implies someone who is ‘between’ the sexes, when in fact people with DSDs are either atypical males or atypical females, not ‘in-between’ or a third sex or no sex at all. 

The term ‘hermaphrodite’ is now considered outdated, derogatory and stigmatising, because:

• It does not describe intersex people correctly.
• Hermaphrodites have sometimes been seen as monstrous.
• It likens intersex people to animals that are hermaphroditic, like snails.

Some people with DSDs have tried to reclaim the term – e.g. when the now-defunct Intersex Society of North America (ISNA) titled its newsletter ‘Hermaphrodites With Attitude’. This is akin to the term ‘queer’ being reclaimed by some gays and lesbians (many other LGB people who have been on the receiving end continue to see it as hateful). Whatever one’s view of that, the least we can say is that it is inappropriate for people without DSDs to use it. 

How many people have DSDs?

Nobody’s right to be treated humanely should depend on how common their condition is. Having data however is important, not least because it is empowering for people with DSDs to know that others share their condition.

The precise frequency of specific conditions varies a great deal. To give a few examples: Klinefelter Syndrome affects around 1 in 1000 births, Androgen Insensitivity Syndrome one in 13,000 births, and Complete Gonadal Dysgenesis 1 in 150,000.

Estimating the total number of people with DSDs depends on which conditions one defines as intersex. The ISNA wrote:

If you ask experts at medical centres how often a child is born so noticeably atypical in terms of genitalia that a specialist in sex differentiation is called in, the number comes out to about 1 in 1500 to 1 in 2000 births.

That would indicate between 0.06% to 0.05% of the population. 

Twenty years ago, Anne Fausto-Sterling et al [7] noted in an influential paper that ‘no well-documented overview of the frequency of intersex exists at present’, and to remedy this they examined the medical literature from 1955 to 2000 to try and estimate ‘the frequency of deviation from the ideal male or female’. Their conclusion was that 1.7% (1 in 60, or 17 in every 1000) of all live births were intersex, where ‘intersex’ means people who do not conform to what they call a ‘Platonic ideal of physical dimorphism at the chromosomal, genital, gonadal, or hormonal levels’. This is a figure for the whole world, as far as the data allows, since the writers looked at international data, not just US data. 

Rather than seeing sex as an absolute of either male or female, they preferred to plot the sexes as bell-shaped curves – which they justified by reference to the wide variety of body shapes, sizes, and sexual characteristics of the sexes – with intersex conditions as a small overlap in-between. Their graph (reproduced below) is popular with people who argue that sex is on a spectrum (and yes, the axes are not properly defined).

This 1.7% figure has been widely reproduced – academically, on social media, and by NGOs, e.g. in Human Rights Watch’s report on intersex child surgery, on UNFE’s Intersex Fact Sheet, or on Amnesty International’s Intersex Awareness page, which says the figure is ‘comparable to the number of people born with red hair’. Likening DSDs to a familiar phenomenon like red hair makes them sound relatively common. The conclusion that is often drawn is that 17 in every 1000 people cannot be easily classified as either male or female.

However, the psychologist Leonard Sax responded that Fausto-Sterling is not using the term ‘intersex’ in the usual way, and that her 1.7% figure ‘includes conditions which most clinicians do not recognise as intersex.’ [8] By including anything that doesn’t fit into a strict ‘Platonic ideal’, Fausto-Sterling makes her definition wide enough to include people whose sex is not at all ambiguous. 

The most common condition that gets put under the ‘intersex’ umbrella, including by Fausto-Sterling, is Late Onset Congenital Adrenal Hyperplasia or LOCAH (not to be confused with Congenital Adrenal Hyperplasia or CAH, which is present at birth), which causes the adrenal gland to produce a slight excess of testosterone in either sex from late childhood or later. In males this is unremarkable, but in females the effects are more noticeable and problematic, such as facial hair growth and infertility. Sax observes: ‘From a clinician’s perspective... LOCAH is not an intersex condition.’ Firstly, intersex is supposed to be about conditions present at birth. Secondly this is a hormonal condition and the sex of the people affected is not ambiguous at all – yet it accounts for 88% of Fausto-Sterling’s estimate (1.5% out of the total 1.7%). 

Leaving out this condition leaves us with a much lower incidence of DSDs, on her figures, of 0.2% of the population. 

Sax argues that deviating from a ‘Platonic ideal’ is not a useful clinical criterion, and that Fausto-Sterling’s definition is still ‘too broad’ and confusing. In fact, the five most common conditions listed by Fausto-Sterling are not intersex in Sax’s view. 

Klinefelter Syndrome for example is a condition where males are born with an extra copy of the X chromosome (making them XXY) in at least some of their cells; but as it affects males, there is no ambiguity regarding their sex. 

He prefers a definition of intersex based upon

conditions in which (a) the phenotype is not classifiable as either male or female, or (b) chromosomal sex is inconsistent with phenotypic sex.

This is a stricter definition than the broad umbrella term I gave earlier. If we define the term in this way and discount the five non-qualifying conditions, writes Sax, the prevalence of intersex on Fausto-Sterling’s own figures emerges as 0.018% (less than 2 in every 10,000). This is very rare, and almost 100 times lower than her estimate – 50,000 individuals in the US rather than 5 million. Put another way, it means that 99.982% of human beings are unambiguously male or female – ‘unambiguous’ meaning that we can identify the person’s sex phenotypically, i.e. simply by looking at their body, without needing to consider other markers such as their karyotype. (Even people with ovotesticular disorder or OT-DSD, who have both ovarian and testicular tissue, develop in the main towards either the male or the female reproductive role, with the other as merely vestigial.)

The intersex advocate Claire Graham makes similar arguments. She criticises Fausto-Sterling’s graph for representing ‘precisely nothing that’s said’ (there are no values on the axes, so what is it charting?) and concludes:

The vast majority of the 1.7% are unambiguously male or female. These conditions prove how robust dimorphism is.

We can see from this disparity the importance of definition. Clearly atypical cases exist, but should the concept ‘intersex’ refer only to cases of ambiguous sex, or to atypical sexual anatomy more generally? If the latter, and your definition of intersex includes unambiguous males and females who happen to be anomalous or untypical in some aspect of their reproductive anatomy, what does ‘intersex’ – a word implying a position ‘between’ the sexes – even mean? There is no straightforward answer. Describing them as a ‘third sex’ (as claimed by US author and activist Hida Viloria for example) is unscientific and meaningless. Sexual reproduction requires male and female gametes: there is no third gamete (what would it even do?), and therefore no third sex. 

These choices are often as political as they are clinical. Since Fausto-Sterling believes that sex is a continuum rather than a binary (see her 2000 book Sexing the Body) and presents DSDs as intermediate values to support her claim, it is in her interest to estimate their numbers as high as possible. 

Natural variation or disorder?

As Dreger argues in her book, intersex people first came under sustained scrutiny in the second half of the 19th century, at the same time as women, homosexuals and sexuality in general were becoming categorised by (male) physicians and scientists. The impulse to assign people to the ‘correct’ box arose from a perceived need to preserve a distinction between male and female, and promote heterosexuality against homosexuality. Unusual bodies were a challenge to the integrity of categories that were seen as natural and inviolable, and so-called ‘hermaphrodites’ were a problem that needed solving. As Suzanne J. Kessler put it in her book The Medical Construction of Gender:

Genital ambiguity is ‘corrected’ not because it is threatening to the infant’s life but because it is threatening to the infant’s culture.

The perceived need to correct category violations has human consequences. The biggest issue facing DSD campaigners to this day is that infants with DSDs are sometimes operated on, with parental consent, to make their bodies conform more closely to expectations. This practice began in the 1950s and 60s in the US under the influence of the psychologist John Money, who thought that intersex people needed their sex disambiguated under his ‘optimal gender policy’. This is the one sense in which we may, in certain cases, talk of sex having been ‘assigned’ at birth. It now happens less frequently, but surgery continues to be carried out on children with atypical sex characteristics. This is opposed by DSD organisations, as it is usually not medically necessary and the child is too young to consent. This is the material consequence of seeing a DSD as an ‘other’ or ‘in-between’ state; in reality, the child is already either male or female. 

 

Recent years have also seen a movement against pathologisation. In Sexing the Body, Fausto-Sterling says that all sexual combinations should be seen as normal:

Complete maleness and complete femaleness represent the extreme ends of a spectrum of possible body types. That these extreme ends are the most frequent has lent credence to the idea that they are not only natural (that is, produced by nature) but normal (that is, they represent both a statistical and a social ideal). Knowledge of biological variation, however, allows us to conceptualise the less frequent middle spaces as natural, although statistically unusual.[9]

Or here is the British LGBTQ+ support charity Outlife:

Being intersex is not a disorder, and is a perfectly normal and healthy way to exist. Intersex variations, though they may be unknown, are relatively common, and cause no harm to your health.

This is simply misleading: some DSDs do cause harm to your health. On their webpage titled ‘What is intersex?’, the US sexual health organisation Planned Parenthood offers a slightly more measured view by including a ‘usually’: 

Being intersex is a naturally occurring variation in humans, and it isn’t a medical problem – therefore, medical interventions (like surgeries or hormone therapy) on children usually aren’t medically necessary.

The issue here is whether DSDs are disorders or just natural ‘differences’ that pose no obstacle to people living healthy lives. It is reasonable to be cautious about classifying people’s given traits as ‘normal’ or ‘not normal’. However, calling for de-pathologisation can be dangerous, because some conditions are pathological. People learn they have a DSD by way of medical diagnosis: if a child is born with unusual anatomy or ambiguous genitalia it should be medically investigated, and certain conditions require ongoing medical management. For example, Androgen Insensitivity Syndrome (AIS) may require hormone therapy, and Salt-wasting CAH can be fatal if left untreated. 

As Claire Graham advises:

Although they are diagnosed as medical conditions, and some can have other health implications, the difference itself is not unhealthy. It does not need ‘fixing’, unless there is a risk to health, impairment of function or the individual chooses to later in life.

That ‘unless’ is important. Morgan Carpenter of Intersex Human Rights Australia writes:

interventions may be necessary for physical health, notably for endocrine issues in congenital adrenal hyperplasia. Surgical interventions may sometimes be necessary to tackle elevated gonadal cancer risks or urinary issues.[10]

People’s medical needs must be addressed on a case by case basis. No one should undergo treatment that is not necessary without their consent; on the other hand, to de-pathologise DSDs entirely would be to deny medical reality and could harm people’s health. It is dehumanising and inappropriate to tell people with DSDs that their conditions do not require intervention when they themselves in certain cases are telling us the opposite. 

Instrumentalising DSDs

One motivation behind the denial of DSDs as a medical issue is – like the exaggeration of their numbers – political. Activists claiming that sex is on a spectrum frequently hold up intersex people as evidence of intermediate stages between male and female. The more common and ‘normal’ people with DSDs are, the more sex looks like a spectrum. 

I have written elsewhere on whether sex is a binary or a spectrum, but it’s important to recognise that having a DSD has nothing to do with being ‘trans’ or with ‘gender identity’. (‘Trans’-identifying people with DSDs are tiny in number, for the simple reason that they are a small demographic within another very small demographic.)

• To have a DSD is to have a physical condition. 
• To identify as transgender is to have a particular attitude to oneself. 

People who identify as transgender are confronted with their own difficulties including discrimination. However there are appropriate ways of supporting them. Intersex advocates are unhappy with their conditions being used to score points for other people’s agendas. (Follow advocate accounts on social media and this will come over strongly.) The Argentinian intersex and trans-activist Mauro Cabral, executive director of GATE, has made this plain:

Intersex is not about being trans, queer or non-binary: it’s about bodies and what happens to people who are born with them. We need to stop instrumentalising intersex to speak our truth as trans people.[11]

In 2013 the Third International Intersex Forum in Malta produced a statement, known as the Malta Declaration, in which it asserted in its final clause:

no-one should instrumentalise intersex issues as a means for other ends.

This statement was drafted by representatives of 30 intersex organisations from all continents. 

The addition of intersex as the ‘I’ to the LGBT acronym might be seen as a way of promoting awareness of people with DSDs. However, it is controversial, as DSDs are a set of physical conditions and has nothing to do with either sexual orientation or gender. Campaigning focused on identity misrepresents DSDs and can harm understanding. Morgan Carpenter observes:

Reports that frame intersex within LGBTI contexts frequently, and incorrectly, often assume that intersex people are all gay, or transgender. From an intersex perspective, the LGBT movement appears preoccupied with identity and relationship recognition, without recognition of bodily autonomy, and deeper challenges posed by sex and gender norms. Discussions within LGBT spaces may suggest that intersex groups do not want inclusion; some intersex-led organisations seek it, while others simply fear becoming overwhelmed and instrumentalised.[12]

It’s debatable whether adding intersex to the LGBT umbrella has brought any gains. The best way to show solidarity with people with DSDs is to promote their actual issues: the first thing organisations can do in this regard, notes Carpenter, is to adopt the Malta Declaration. 

Conclusion

People’s sex is important, as it is a material fact about their bodies that has implications for their lives: above all their medical needs and reproductive role, which are different for either sex. It also of course results in social distinctions being made which can be controversial or outright harmful. 

Because concepts like ‘male’, ‘female’ and ‘intersex’ are (like all words) human linguistic and social constructs, and inevitably have fuzzy edges, it can be tempting to query their value. When Alice Dreger says ‘there is no... final answer to what must count for humans as “truly” male, female, or hermaphroditic [sic]’ [13], this is true up to a point. However self-evident our concepts seem to us, they are unavoidably plastic and historical, since our societies, knowledge and technology are mutable. For example, the current understanding of ‘sex’ as a biological term and ‘gender’ as a social/cultural term only appeared in the 20th century – and if you advocate gender identity theory you will understand them differently again (and even, on some readings, flatten the two into each other).

On the other hand, there is a material, biological reality which these concepts are attempting to grasp, by reference to which 99.98% of human beings fit unambiguously into our concepts of male or female. Even the very concept of ‘intersex’ confirms the binary that people with DSDs are put into relation to. Only a tiny 0.018% proportion of people trouble those concepts, and even those can be categorised as male or female by assessing relevant criteria. This tiny amount of ambiguity tells us our sex categories are remarkably robust, compared to the concept of the colour ‘orange’, for example, whose edges are very fuzzy indeed. For them to be meaningful, it is not necessary to have two ‘perfect’ concepts with zero ambiguity – such a demanding standard is not required of other concepts. 

What people with DSDs need is the support of their families and peers, and properly funded healthcare provision to meet their medical needs. People should make choices about their own bodies, so seeking legislation to protect infants from non-consensual and medically unnecessary surgery is one of the movement’s priorities. 

In summary:

• The idea that any human being can be ‘between’ sexes is misleading. There is no middle condition. People with DSDs are still either male or female, the great majority without any ambiguity; there are some very rare conditions where determining sex is more complicated. 
• If we leave out the late-onset condition LOCAH and keep to congenital conditions, the incidence of DSDs in the population on current evidence is not the oft-quoted 1.7% but 0.2% (1 in 500).
• Cases where sex is ambiguous are extremely rare: 0.018% of births (less than 2 in every 10,000).
• The existence of DSDs does not mean that humans are not sexually dimorphic or that the sex categories of male and female are unstable, beyond an expected, empirically observable and measurable variation.
• Since there is no third sex (let alone more), sex is binary. This doesn’t mean human behaviour or gender expression or sexual attraction or identity is or ‘ought’ to be binary.
• It is necessary to tread a responsible line between overstating and understating DSDs as ‘disorders’ rather than differences: each person’s case must be judged on his or her own needs.
• People with DSDs have their own needs and priorities and should be not instrumentalised by the political agendas of others. 

---

Footnotes

[1] Alice D. Dreger, Hermaphrodites and the Medical Invention of Sex (1998), p3-4.

[2] Richard Goldschmidt, ‘Intersexuality and the Endocrine Aspect of Sex’ (1917).

[3] Ovid, Metamorphoses, Book IV, translated from the Latin by Mary Innes. 

[4] These terms can get confused. Hermaphroditism (the anatomical possession of the characteristics of both sexes in one individual; impossible in humans) is distinct from androgyny (an appearance as neither or both genders, natural or cultural or a mixture of both; possible in humans). 

[5] Diodorus Siculus, Bibliotheca Historica (Historical Library), 1st century BCE, Book IV 4.6.5. 

[6] Dreger, Chase, Sousa, Gruppuso, Frader, ‘Changing the Nomenclature/Taxonomy for Intersex: A Scientific and Clinical Rationale’ (2005).

[7] Anne Fausto-Sterling et al, ‘How Sexually Dimorphic Are We? Review and Synthesis’ (2000). You can read the whole article at this link. 

[8] Leonard Sax, ‘How Common is lntersex? A Response to Anne Fausto-Sterling’ (2002). Although I find Sax’s work useful here, he also has reactionary views on sex differences. 

[9] Anne Fausto-Sterling, Sexing the Body: Gender Politics and the Construction of Sexuality (2000), p76. 

[10] Morgan Carpenter, ‘The human rights of intersex people: addressing harmful practices and rhetoric of change’ (2016).

[11] Mauro Cabral Grinspan, 2016.

[12] Carpenter, op. cit. 

[13] Dreger, op. cit., p9.

What are hormones?

Besides producing sex cells, another function of the gonads is to secrete hormones. Hormones are chemical molecules found in all multi-cellular organisms. They are secreted by the body’s endocrine glands directly into the blood, which carries them around the body so they can perform their tasks. Hormones are best thought of as messengers that carry instructions to our tissues and organs: an internal communication system between different parts of the body, making its cells do certain things. There are many kinds of hormone, affecting such things as our growth and puberty, sleep, cognition, metabolism, and sexual reproduction. The endocrine system therefore is a conversation in the body between glands, hormones and cells.

The endocrine system is made of various glands scattered around the body, the major ones besides the gonads being:

- Brain: hypothalamus, pituitary gland, and pineal gland
- Neck: thyroid and parathyroid glands
- Between the lungs: thymus
- Above the kidneys: adrenal glands
- Behind the stomach: the pancreas

The glands produce the hormones, the hormones travel around the body in the bloodstream (usually), and the hormones bind to receptors on the target cells. This hormone/receptor union triggers processes in the cell. By exposing millions of cells to regulated quantities of different hormones, the endocrine system can orchestrate changes in the body.

Hormonal change can also provoke mood changes, because they can affect the quantities of chemicals like seratonin that influence how we feel.

Human sex hormones are steroid hormones, which mean they can pass through the fatty outer membrane of their target cells, enter the nucleus, and bind to its DNA, and can therefore influence gene expression. (You may also have heard of anabolic steroid hormones, which are artificial molecules that mimic the actions of testerone.)

Males and females produce the same hormones: the difference lies in the relative quantities of each.

Male

The testes secrete a group of sex hormones known as androgens, which play a role in producing male sexual traits. They are produced by women’s ovaries too, but in much lower quantities.

The major androgen is testosterone (produced primarily by the testicles but a bit is also produced by the adrenal cortex). Things governed by testosterone include:

• Growth of male reproductive organs
• Heavier skeleton and musculature
• Growth and distribution of body hair e.g. beard
• Larger larynx and thus deeper voice
• Sex drive
• Sperm production

The production of testosterone begins while the foetus is developing in the womb, and continues for a short time after birth. During childhood it virtually stops until resuming at puberty, the point where boys really open up the physical gap with girls. Then it starts to decline after the age of 30 or so. As there is no male menopause, there is no sudden fall in testosterone production equivalent to the fall in women.

Female

As well as producing eggs (oocytes), the ovaries secrete the hormones oestrogen and progesterone, both of which govern female sex characteristics amongst other things. Men’s testes (and adrenal gland) produce them too, but in much lower quantities.

Oestrogen is technically a group of hormones, the three major ones being estradiol, estrone, and estriol. It helps develop and maintain both the reproductive system and female sex characteristics, such as the breasts, but also affects more general health such as the bones and metabolism.

Progesterone is particularly associated with preparing the uterus for pregnancy, but both hormones play a role in pregnancy and the menstrual cycle (there are also other hormones that contribute).

• Growth of female reproductive organs (uterus, vagina)
• Growth of breasts
• Widening of the pelvis
• Distribution of body fat
• Sex drive

Production of oestrogen and progesterone increases during puberty. During the menopause, i.e. when a woman stops being able to give birth to children and her periods cease, the levels of these hormones fall fast.

Sex hormones

Contrary to common misconception, there are no exclusively ‘male’ or ‘female’ sex hormones. Both sexes produce testosterone, oestrogen and progesterone, just in differing quantities.

Sexual development and reproduction in human beings

In this article we’ll continue to examine biological sex, and look more closely at how sexual reproduction works in human beings. 

Male and female development

As a sexed species, humans develop as either male or female. 

The reason sex exists is to serve as a mechanism for producing young. Humans are anisogamous, i.e. our method of sexual reproduction involves the fusion of a small gamete with a large gamete. Thus the sexes are defined by the gamete type that their reproductive anatomies are organised around. Male bodies are organised around the production of small gametes (sperm); female bodies around the production of large gametes (eggs). As there only two gametes, there are two sexes, and our species is sexually dimorphic (from ancient Greek meaning ‘two forms’). 

There is of course enormous variation within these two types, creating a spectrum of body types. This is sometimes claimed as evidence that sex is not binary or even that sex itself is on a spectrum. But it is incorrect to define the sexes in terms of these variable bundles of characteristics. Sex is not defined by chromosomes, hormones, or secondary sexual characteristics but by gamete type. 

The sex of a human being is decided at conception, i.e. when the sperm fuses with the egg to form a zygote with a complete genome. The egg always supplies an X chromosome. The sperm provides either an X or a Y: a 50% chance of each. In 99.8% of cases the resulting zygote has the typical sex chromosomes (so-called) of either XY (male) or XX (female) and goes down a typical pathway of phenotypic development. People with rare DSDs (the other 0.2%) may have atypical karyotypes or phenotypes but are still male or female; to claim they are neither, or some mixture of the two, is false and dehumanising. 

Sex differentiation

No sexual differences can be observed in the fetus until the sixth or seventh week of gestation. The key to sex determination is the activation or non-activation of the SRY-gene on the Y chromosome (though other genes also come into play). If/when this gene is activated, the fetus becomes male and develops testes; in the absence of SRY activation the fetus becomes female and develops ovaries. For this reason biologists sometimes call the SRY gene a ‘master switch’ since the fetus will develop into a female unless the gene is activated. 

Thus chromosomes determine sex – they supply the instructions for how it should develop – but are not the same as sex. We can clarify this with a couple of examples of DSDs:

• If the fetus has XXY, the presence of the Y chromosome still leads to SRY activation and the fetus becomes male. The variation in karyotype is not a new sex; it merely produces an atypical male. 
• A fetus with XY in whom the SRY gene does not activate will develop as a female. Despite her XY chromosomes she is not a male but an atypical female. 

Note this process also depends upon the fetus’s cells being able to respond to androgens (the male sex hormones required for male sexual development). If the cells are insensitive to those androgens (as in the DSD known as Complete Androgen Insensitivity Syndrome or CAIS, in which cells’ androgen receptors are rendered inactive by a mutation) then the fetus won’t respond to the masculinisation process and will become female despite having XY chromosomes. 

In summary: to become male, a fetus needs 1) a Y chromosome with an activating SRY-gene in combination with 2) functioning androgen receptors. Failing this, the fetus will be female.

Pathways of development

The sex of an individual is defined by his or her pathway of development. Before sexual differentiation, an embryo possesses both Müllerian and Wolffian ducts: precursor structures that will develop into the reproductive organs. [1] Once sex differentiation kicks in, it retains one and loses the other. 

• Müllerian ducts develop into the female reproductive system, organised around large gametes (eggs)
• Wolffian ducts develop into the male reproductive system, organised around small gametes (sperm)

These two pathways of sexual development are mutually antagonistic and cannot functionally co-exist, as humans are not hermaphrodites. If the embryo is sent down the male pathway, it produces anti-Müllerian hormones that actively suppress the female structures. 

Even if a given person’s reproductive anatomy is not fully developed or fully functional, e.g. s/he is infertile and does not actually produce any gametes, they are still male or female as their body developed down one pathway or the other. It would be most offensive for example to tell a woman who was born without a cervix, or has had her ovaries removed due to cancer, or has gone through menopause and thus no longer produces eggs, that she is therefore not female. 

Gonads

Gonads are the primary reproductive glands, found in all animals that reproduce sexually. In human beings, the male gonads are the testes, a.k.a. testicles, and the female gonads are the ovaries. The purpose of these organs is to produce the reproductive cells or gametes (sperm in males, eggs in females) that are required to create new life. The gonads can take variable forms according to species, but in higher vertebrates they are both binary (either male or female per individual) and permanent.

They also secrete hormones responsible for the development of primary and secondary sex characteristics, and are thus considered to be part of the endocrine system. I’ll discuss that separately.

Testes

The testes are a pair of oval organs that have two functions: to produce sperm, and to produce hormones, especially testosterone.

In humans they are descended, i.e. they hang between the legs in a sack of skin called the scrotum. This is to distance them from the rest of the body, because sperm production requires a slightly cooler temperature, about two or three degrees Celsius lower (it’s not known for certain why, or how the curious phenomenon of descended testicles evolved).[2]

Sperm are created (the scientific term is spermatogenesis) in a system of tightly coiled tubes called seminiferous tubules, where cells known as germ cells divide and develop to form organisms resembling tadpoles, with a head and tail – namely sperm. The process takes about three months. Afterwards the sperm are stored for a few days in a long, coiled tube behind the testicle called the epididymis where they mature and learn to swim. Once mobile, they are ready to attempt fertilisation.

Boys start producing sperm when they hit puberty (on average around age 13), a milestone called ‘spermarche’ for what it’s worth. A man produces millions of sperm cells a day, and about 525 billion over his lifetime.

Ovaries

The ovaries, too, are a pair of oval organs with two functions: again, to produce gametes and to produce hormones. This time, the gametes are eggs (or ova, singular ovum) and the hormones are oestrogen and progesterone.

The term ‘egg’ is a little vague and gets used for different stages of the process. To be precise, the ovaries produce oocytes, or immature egg cells. These oocytes are created from germ cells by a process called oogenesis which takes place when the baby girl is still in the womb. Most of these 7 million oocytes die before she is even born. Whereas the testes produce new sperm continuously, females are born with about two million oocytes, which in principle is the maximum number of eggs she can produce in a lifetime. By puberty she is already down to about 300-400,000, of which maybe 300-500 will actually get ovulated during a woman’s reproductive lifetime. The number and quality/viability decline with age.

The oocytes are found in structures inside the ovaries called ovarian follicles. There are hundreds of thousands of these, the great majority of them with an oocyte in the centre. Once a month an oocyte breaks out of its follicle as part of ovulation, which we’ll get to in a second.

Meiosis

The gonads create gametes through a process of cell division called meiosis.

A cell has a life cycle: it comes into existence when it divides from a so-called ‘mother’ cell, it reproduces by dividing into two, and at some point it will die or be replaced.[3] In a process called mitosis, a cell divides itself into two identical cells, with an exact copy of its DNA in each. All the cells involved are ‘diploid’.

• Diploid: a cell with two sets/copies of chromosomes in the nucleus (46 in humans).

Meiosis is a bit different and more complex, as a gamete by definition has only half the genetic information. So meiosis needs to halve the 46 chromosomes to 23. This division requires a two-step process (we needn’t go into detail about the reasons why, but they are explained in this video if you are interested). Whereas in mitosis the original diploid cell divides into two new diploid cells, in meiosis it divides into two new ‘haploid’ cells.

• Haploid: a cell with just one set/copy of chromosomes (23 in humans). Gametes (egg or sperm cells) are the only haploid cells in the human body.

Then the two new haploid (23-chromosome) cells themselves duplicate, mitosis-fashion, into two further haploid cells, and these are the gametes. Thus one starting cell can produce four gametes.

Meiosis therefore serves just one purpose: to produce gametes, with only half the chromosomes of the original cell, i.e. a reduction from diploid to haploid. To return to the diploid state, a male and a female gamete must fuse via fertilisation. 

An important process that takes place during meiosis is recombination. This is the reshuffling process by which, when creating the gamete’s genetic material, each chromosome pair swaps pieces from one chromosome with pieces from the other chromosome. Thus a father and a mother each has material from both their parents in their gametes. In this way recombination ensures genetic diversity: a child created by the father and mother inherits their genetic material, which thanks to recombination will include genetic material from all four grandparents. The child does not however receive carbon copies: there is still some variation, which means two siblings do not inherit the exact same DNA unless they are identical twins.

Gametes

Gametes therefore are the haploid reproductive cells or sex cells, which come from germ cells. In humans, once again,

• the male gametes are sperm produced by the testes
• the female gametes are eggs produced by the ovaries

Both are incredibly tiny single cells that each contain only half the genetic information required to create a new human being. 

The fact that each gamete contains half means that there need only be two. This is why there are two sexes. No third gamete exists, or could have any role to play, and thus there is no third sex (let alone a fourth, fifth or more). Although many concepts in biology have fuzzy boundaries (notoriously the concept of species), there is no room for fuzziness with gametes. One simply produces one or the other (or in some cases, none at all). There are no intermediate forms. 

Sperm

Sperm is short for ‘spermatozoa’, singular ‘spermatozoon’. It is an independent, single-celled living organism, about 0.05mm long, making it much too small to see with the naked eye. The plant equivalent is pollen.

The sole purpose of the tiny, torpedo-like sperm cell is to deliver a genetic package to fertilise an egg. The DNA is carried in the nucleus in the sperm’s head. It has some mitochondria in the middle to provide the energy it needs to swim through the female’s body, and it is propelled by a tail.

During sexual activity, the sperm are mixed with a white-ish fluid to form semen, which allows the sperm cells to be propelled up the urethra (the tube inside the penis, the same tube pee comes out of), and travel into the female’s body. The penis is a delivery mechanism to get the male’s gametes within travelling distance of the female’s.

Egg

Whereas the male strategy is to hurl out a vast number of cheaply produced contenders in the hope that one of them wins the prize, the female strategy is to make far fewer, much more expensive investments. The round egg cell is 10,000 times the size of a sperm cell. At about a tenth of a millimetre it is just about visible to the naked eye – about the size of a full stop. This makes it a giant among cells, and the largest in the human body. This is to maximise the likelihood of the offspring surviving: while the sperm only passes on its nucleus, the egg cell has to provide enough energy for the new human being it will develop into.

The core of the cell is the nucleus, which contains the 23 chromosomes. The cell’s structures are held together by a kind of gel called cytoplasm. Then there are some outer layers: a cell wall or membrane called the zona pellucida, then a surrounding layer called the corona radiata that provides proteins to the cell.

We have seen that gametes are created by meiosis and that meiosis has two stages. The oocytes enter the first stage of meiosis before the girl is born, then stop before completing it. They are still stuck at this point when the girl is born, and stay that way until puberty.

Once the girl hits puberty her reproductive cycle begins, or what is actually two concurrent cycles: firstly the ovarian cycle which matures and releases eggs, and secondly the menstrual or uterine cycle, which prepares the uterus to handle any eggs. One egg is activated each month in a process called ovulation: some of the ovarian follicles along with their oocytes will begin to grow, until one of them wins the race by growing faster than the others. That oocyte finally completes the first stage of meiosis and breaks out of the follicle, leaving the ovary to travel down the fallopian tube, which serves as a passageway to the uterus. 

NB: the oocyte is not to scale.

If the egg gets fertilised, it undergoes the second phase of meiosis to become, briefly, an ovum (i.e. a mature female gamete or sex cell) – but only until the sperm nucleus fuses with the egg nucleus, whereupon it becomes a ‘zygote’. The terms can be a bit confusing, because technically the cell is an ovum for barely any time at all, but you will often hear the term ‘ovum’ used more loosely, and ‘egg’ more loosely still.

After ovulation the egg lives for 12-24 hours. It may or may not get fertilised by a sperm during that time, but either way, it travels on into the uterus.

• If fertilisation did not happen, the egg disintegrates and is ejected in menstruation.
• If fertilisation does happen, the egg implants itself in the lining or wall of the uterus where it will develop, eventually, into a baby.

Fertilisation

Since gametes are haploid cells with only half the genetic information required to make a new human being, two (one male, one female) have to fuse together to complete the set. This is the role of sexual intercourse: to bring gametes from two individuals together. The successful joining of the sperm and the egg is called conception. 

Once ejaculated during intercourse, sperm swim up the vagina, through the cervix (the opening of the uterus), and through the uterus to the fallopian tube where the egg awaits. This is a relatively long and hazardous 14-hour journey, where the sperm’s swimming is heavily assisted by contractions of the uterine muscles. The sperm need to push through the corona radiata and zona pellucida to attach to the egg. The first sperm to make contact will fertilise the egg. Out of the millions of sperm released into the woman’s body during sex, only one can succeed.

Once a sperm succeeds, the egg releases proteins and enzymes that ensure that no other sperm is allowed in, and the two gametes fuse within minutes. 

Fertilisation is the key moment in sex: the moment when the genetic material of the male and female parents is combined or fused. This is the whole point of sexual as opposed to asexual reproduction. In asexual reproduction, as we’ve discussed, there is only a single parent and thus no fusing of gametes i.e. of genetic material.

The genetic material of the sperm uncoils to form a male pronucleus inside the egg, i.e. at this point there are two (pro)nuclei, one from either cell. The egg at this point completes meiosis. The genetic material of the two nuclei joins together, completing the fertilisation process: a unique genetic code is created, deciding the characteristics of the new human being.

This includes its sex. As I mentioned above, sex is determined at fertilisation, because that is when the 23rd pair, the ‘sex’ chromosomes, are determined as (normally) either XX (female) or XY (male). However, the fetus does not develop its sexual characteristics until it is about seven weeks old. By the time a baby is born, its phenotype is (again, normally) fully male or female.

Chromosomes

Let us now return to the subject of chromosomes.

X chromosome: the X chromosome contains information essential to both sexes – at least one copy must be present in all human beings. It contains about 800-5000 (depending on who you ask) of the human genome’s 20,000+ genes. It’s generally said to contain 5% of a person’s total DNA.

Y chromosome: the Y chromosome is male-determining. It has about 55-78 genes (depending on who you ask), and only about 27 unique ones, most of which are related to creating sperm. The key gene is called SRY. If you have this gene and it activates, you develop into a male. If you don’t, or you do but it doesn’t activate, you develop into a female.

The egg always carries an X chromosome, so sex is determined by the sperm: 50% carry an X chromosome and 50% a Y, so when the gametes fuse, the zygote will end with either an XX pair or an XY pair. Everyone therefore inherits one X from their mother, plus either a Y or an additional X from their father.

Zygote

The fertilised egg now contains 46 chromosomes, half from the sperm (father) and half from the egg (mother), and thus contains all the genetic instructions needed to make a baby. This new, diploid kind of cell is called a zygote – the beginning of a new human being.

This cell then divides multiple times and goes through various named stages (I won’t list them all). First it divides into two cells to become multicellular, i.e. an embryo, then the embryo develops into a foetus and eventually (after nine months) a baby.

Conclusion

I am not aware of any peer-reviewed paper in the entire scientific literature that disputes that there are two gametes and therefore two sexes. If someone could produce proof that humanity has been wrong for thousands of years and that there are in fact more (or less) than two sexes in human beings, it would be one of the most astonishing scientific discoveries of all time. 

---

Footnotes

[1] Fei Zhao, Humphrey Hung-Chang Yao, ‘A tale of two tracts: history, current advances, and future directions of research on sexual differentiation of reproductive tracts’ (2019).
[2] This incidentally means that the testes – unlike the ovaries which are safely tucked away inside the body – are exposed and vulnerable, putting a male’s genetic legacy at risk, which is why they have extra pain-sensitive nerve endings to ensure their owner protects them.
[3] You will sometimes read that the human body replaces itself every seven years, but the seven years figure is meaningless. Cells die and are replaced all the time.

What is sex?

The English word sex can be traced to the late 14th century and comes, via French, from the Latin sexus which means (only) the state of being either male or female, possibly deriving from the root sec- meaning ‘cut’ or division. Since then, ‘sex’ has meant

either of the two main categories (male and female) into which humans and many other living things are divided on the basis of their reproductive functions (OED). 

It is a biological category that divides people according to their genetics as well as sex characteristics such as reproductive organs. Nobody has any choice in what sex they are born as. They pop out into the world as either one or the other [1], and must deal with that as they will.

There are processes that exist in the material world, and then there are the concepts (or universals) that we humans abstract from the material particulars in order to think and talk about them: ‘sex’, ‘male’, ‘female’, etc. These concepts are culturally mediated, but changing the words, or changing the way we think about and categorise sex, will make no difference whatever to the material facts. Those exist regardless, independently of human thought. 

As a concept sex seems simple enough, until you look at it closely. Some of what I’ll cover below is a bit technical, but as with genetics, it’s important to ground ourselves in facts before we get onto the social and philosophical questions.

Methods of reproduction

No living organism is immortal, so for a species to keep existing, it has to reproduce, i.e. to create new generations that will live on when the older generations die. Natural selection has produced, over billions of years, a rich diversity of methods, but all can be categorised as either asexual or sexual. All bacteria reproduce asexually, most animals reproduce sexually, and some species can adopt either method, depending on the circumstances (e.g. wasps, starfish, aphids, komodo dragons).

In essence, sex is an evolved mechanism for the reproduction of species. 

Asexual reproduction

In asexual reproduction, one organism makes a clone of itself. Bacteria, some plants and even some animals (e.g. some sponges and sea anenomes) reproduce this way.

There are several methods of asexual reproduction. We needn’t look at them in detail, as my concern here is with human beings, but very briefly, the four main types are:

• Binary fission:  meaning ‘division in half’, this usually occurs in bacteria. A single parent cell splits into two, copying its DNA into the new cell.
• Budding: the parent organism grows a bud or outgrowth which develops into a new organism, which may or may not separate. Used by some plants and some animals. 
• Parthenogenesis: meaning ‘virgin birth’, this is reproduction from an egg without fertilisation by sperm. Used by some plants and some animals.
• Fragmentation: part of the parent organism breaks off and then develops into a new individual. This is different to budding because the new organism begins as a separate piece to the parent, not just an outgrowth.

What the methods have in common is that they allow one individual to churn out offspring all by itself, with relatively little expenditure of energy.

Sexual reproduction

In sexual reproduction, two parents of the same species – in anisogamous species (see below), one male, one female – combine their genetic material to create offspring that inherits a mixture of its parents’ genes and will therefore be slightly different to both.

Sexual reproduction requires the fusion of the two parents’ genetic material in the form of gametes, or sex cells. In animals, the male gamete is the sperm and the female is the egg, and fertilisation happens when the two come together. Fertilisation doesn’t necessarily involve sexual intercourse – and thanks to the technology of artificial insemination, this can now be true even in human beings.

The two parents don’t always have to be separate individuals: hermaphroditic organisms (e.g. earthworms) can supply both male and female gametes, and self-fertilise. Since two gametes are required, however, this is still considered sexual reproduction.

In summary, there are two models for organisms to reproduce themselves:

Asexual	Sexual
No sexes, one parent	Two sexes, two parents
Individual makes a copy or clone of itself	Two gametes whose fusion leads to fertilisation
Offspring genetically identical to parents	Offspring genetically variant to parents
Quick and easy, but no genetic variation	More resource-costly, but gives genetic variation

Of course there are exceptions. Some plants and fungi can do either asexual or sexual reproduction depending on the conditions. Even some animals can do this, via parthenogenesis. There are plants and a nematode worm that are trioecious, i.e. the organism has three forms – male, female, and bisexual or hermaphrodite. However the latter possesses both sex parts at once, so even this ‘third sex’ is built upon the male-female binary.

Evolution is conservative. Sexual reproduction involves two parents, and two gametes. 

Isogamy and anisogamy

The vast majority of sexually reproducing species are divided into ‘male’ and ‘female’, defined by the size of the gametes they produce:

Male: small gametes

Female: large gametes

This is the textbook, scientific definition of male and female. 

This dichotomy in gamete size, which is found in almost all complex multicellular eukaryotes [2] (a group which humans belong to), is known as anisogamy. When the differential in size is accompanied by a differential in motion where the small gamete is mobile and the large gamete is immobile, this more extreme form of anisogamy is known as oogamy, which is found in almost all animals. This dichotomy leads to varying degrees of sexual dimorphism, i.e. when males and females have different body types, as in our own species. 

In many single-celled organisms, sexual reproduction occurs without a dichotomy in gamete size (and thus also no dichotomy of sexual dimorphism). This is known as isogamy, and the outcome is the same as in anisogamy, i.e. the fusion of the two gametes leads to fertilisation. However, because the gametes in such species are the same size, we don’t classify them as male and female. Instead, isogamous organisms use a system of mating types. Most commonly there are two, called + and - (plus and minus), though there can be more. 

Isogamy is thought to be the ancestral form from which anisogamy evolved – the origin of male and female lies in this transition. In isogamy the parental investment is equal, whereas in anisogamy the female invests more resources in each gamete than the male, and the potential appears for natural and sexual selection to produce morphological differences between the sexes. 

Overwhelmingly, organisms have either no sex or two sexes. 

Why have two sexes?

Sexual reproduction is more costly in time and energy. It requires organisms to find and court a mate, involves lengthy gestation, and demands parental care, and therefore constrains reproduction. Asexual reproduction is quicker and easier: one aphid for example can have 600 billion offspring in one season.

So why have two sexes? The most likely explanation relates to genetic diversity.

In asexual reproduction, because there is no other parent, i.e. no other source of genetic material, every new generation will be genetically identical to the previous one. (Genetic variation does arise, but through genetic mutation.) This works fine when conditions are stable, but it can potentially put the species at risk.

• Each generation has the same vulnerabilities. If a disease appears that can kill one individual, it can kill the entire population. 
• Each generation depends upon the same habitat, or at least there must be no change in habitat that will harm the species’ survival. If the selection pressures change, the population will adapt much more slowly. 

Sexual reproduction by contrast relies upon the combination of genetic material from different parents, which over several generations can result in a lot of variation.

• Each generation has genetic variation within the population. If a disease appears, there is more chance that some individuals will have resistance and survive. 
• If habitat changes, there is more chance that some individuals will have traits that allow them to adapt.

These are two strategies developed by evolution to ensure the survival of organisms. The fact that organisms of both kinds still exist tells us that both strategies have (so far) been successful in perpetuating species. No one method of reproduction is ‘superior’ to another: there are simply different strategies that have proved successful for certain species under certain conditions.

The answer, then, to the question ‘what is sex?’ is – from a narrow biological perspective – that sex is a means for certain species to reproduce themselves by fusing male and female gametes to ensure genetic diversity. In the next article I’ll look at how this works in our species, Homo sapiens.

---

Footnotes

[1] Even people with DSDs are basically either male or female, unless they’re one of the tiny percentage whose sex is ambiguous. I discuss intersex/DSDs separately here. 

[2] Jussi Lehtonen, Hanna Kokko, and Geoff A. Parker, ‘What do isogamous organisms teach us about sex and the two sexes?’ (2016).

A short introduction to genetics

This article may seem like a digression from our investigation of sex and gender, but it isn’t possible to sustain an authoritative discussion without at least a basic understanding of the relevant aspects of genetics, such as what chromosomes are. This is just a short, simple introduction.

Of course, science does not claim to fully understand this staggering achievement of evolution any more than I do.

DNA

All genetic material is made from a chemical called deoxyribonucleic acid, or DNA for short.[1] This is the code of life. The information is stored as a code made from four basic building blocks, or chemical bases (known as nucleotides) which we render as letters: adenine (A), guanine (G), cytosine (C), and thymine (T). These pair up with each other to form units called base pairs. The order, or sequence, of these bases provides the instructions for how an organism should be made, like a recipe of the traits we inherit from our parents. In humans there are roughly 3 billion bases – the number is different for other species.

Credit: US National Library of Medicine

Structurally the DNA molecule is made of two strands that wind together to form the famous ‘double helix’ or double-stranded spiral. This remarkable, elegant structure is shared by all living organisms.[2] The structure resembles a twisted ladder: the base pairs are the rungs, and the two long strands are the uprights.

The advantage of this ladder structure is that it’s highly stable. This is important because when cells divide, the DNA copies itself to the new cell. An exact copy of our DNA is thus found in the core or nucleus of every cell in our body, i.e. each cell contains our entire genetic code. This copying ability allows all known living organisms to grow and reproduce. Cells are the basic structural unit of such organisms (the human body is estimated to have over 30 trillion), and the nucleus controls the processes of the cell.

A small amount of DNA also appears in the mitochrondria (structures within cells that convert energy for the cell to use): this mitochondrial DNA is useful for tracing our (maternal) ancestors but needn’t detain us here.

Genomes

A genome is an organism’s full set of genetic instructions, which tell it how to grow and develop – the total of all its DNA. It is the complete set of the billions of ATCG letters that make you what you are. The complete human genome was mapped in 2000, helping us to explore how genetics works, for example researching the causes of genetic diseases.

Different species vary because each has its own distinct genome, but every individual within a species also has (smaller-scale) variations that give it its own particular configuration. Apart from identical twins, everyone’s genome is unique; we inherit mutations from our parents, but we also have our own mutations, combinations which may never have existed before.

These mutations or glitches – a bit like a spelling mistake – can result in our cells receiving the wrong instructions. This might work to our advantage, or it might confuse the instructions and cause something not to work properly, possibly even with fatal consequences.

Chromosomes

There is an immense amount of information contained within the spiral, and the DNA in each cell would stretch to two metres long if unravelled. That’s a lot to pack into a tiny cell. So evolution has devised a solution: it divides the DNA into (in humans) 46 sections and coils each one into a structure we call a chromosome (along with some proteins that help to stabilise and package it).

Chromosomes are usually pictured in a characteristic X form, but a chromosome only really looks like this during cell division, when it condenses to avoid getting tangled, and has a copy of itself attached. For most of a cell’s life chromosomes look more like string.

The number of chromosomes in the nucleus of a cell is called ploidy; the condition of chromosomes existing in pairs is diploidy; the condition of having an atypical number of chromosomes, i.e. some are missing or extra, is called aneuploidy. The number varies by species (and is not, by the way, a measure of complexity) – fruit flies for example have only 8, dogs have 78 and goldfish have 94. 

Humans are diploid organisms. Each human cell contains 46 chromosomes [3], organised into pairs: we humans have 23 types of chromosome, and two of each type. One of each pair is inherited from our father, the other from our mother. In other words, we inherit half our DNA from either parent: 23 chromosomes from each. This is why we can share some characteristics from our father and others from our mother.

All chromosomes do not look the same: there are several types based on variations in their structure.

22 of these pairs are called autosomes and are the same in both males and females. The 23rd pair, the one of particular interest to us here, is different: the so-called ‘sex chromosomes’. There are two types of sex chromosome, either X or Y.

• Females have two X chromosomes
• Males have one X chromosome and one Y chromosome
• These are the two normal patterns. However, other combinations can be found

An individual’s collection of chromosomes is called a karyotype (‘carry-o-type’). This is also a lab technique that produces an image of the chromosome pairs lined up like in the example below, so they can be checked for abnormalities. Pairs 1-22 are autosomes, pair 23 are the sex chromosomes.

Although female embryos inherit one X chromosome from each parent, in every cell one of the two is ‘switched off’ and is not expressed. This could be either the maternal or the paternal X; in males, the X chromosome is always from the mother.

The X and Y chromosomes are often described as ‘sex chromosomes’ (I’ve done it here myself) but we shouldn’t think of X as female and Y as male, because men have an X too, and thanks to rare DSDs like CAIS it’s possible for females to have Y chromosomes. Also, note that sex is produced by a number of genes interacting in different ways at different stages of development: it is not the product of the X and Y chromosomes alone. 

Chromosomes, then, determine sex but are not synonymous with sex. Karyotypes are not sexes. Having variations in your karyotype – such as XXX (Triple X syndrome) found in about 1 in 1000 females, or XXY (Klinefelter syndrome) found in 1 in 1000 males – does not mean you are a ‘new’ sex. You are just a male or female with an unusual karyotype. 

I shall say more about chromosomes when we discuss human sexual reproduction.

Genes

A gene is a small segment of DNA – a section of a chromosome – that is the basic unit of heredity. Humans are estimated to have about 20,000-100,000 genes, depending on who you ask (they are hard to count and we don’t know for sure). Each of us has two copies of each gene, one inherited from each parent, and while most are the same, a small number (less than 1 per cent) are different. These are known as alleles (‘a-LEELs’), i.e. variant forms of the same gene that allow for the differences in people’s physical characteristics. We’ll talk about those in a moment.

If DNA is a recipe for making an organism, genes are sub-units that contribute the specific ingredients. They convey the information for setting up particular physical traits: whether we have light or dark skin, blue or brown eyes, whether we are short or tall, and so on. Such characteristics may be determined either by a single gene or by several genes in interaction. (Bear in mind however that our traits can also be determined by our environment – genes do not act in isolation from the world.)

How this works is that genes tell cells how to build proteins. Cells need to multiply in order to build up into a new young creature’s body parts: each time a cell multiplies, it copies itself along with the DNA in its nucleus. Then the cells read the instructions in the DNA (this is called gene expression) to make proteins, substances that are essential to structuring and regulating the tissues and organs that make up a living body. Proteins are made from building blocks called amino acids. A gene codes a series of 20 amino acids into one of thousands of possible sequences to produce different proteins that fulfil different functions.

Chromosomes contain the genes that code for the proteins that make up our bodies

Most DNA however (called non-coding DNA) doesn’t code for proteins: i.e. all genes are DNA, but not all DNA is genes. In fact genes only make up 1-5% of your genome. Most DNA is not genes, and instead controls other things like gene expression and a process called gene regulation or gene switching. Each cell performs a different role in the body, so individual cells express (or ‘turn on’) only a fraction of their genes. Genes are switched on or off depending on the intended function of a cell: in eyeball cells, only the eyeball genes are turned on, etc.

This is how the DNA recipe builds a wide range of particular parts into an entire living organism.

Genotype vs phenotype

Sex can be divided into genotype and phenotype. These work together to give people their particular characteristics.

• Genotype is an organism’s genetic constitution: the information or code contained in its genes that determines its physical traits. 
• Phenotype is the expression of the genotype, i.e. the observable physical traits that result.

Genes contain all of your options for how a physical trait might be, passed down by your parents. For each trait, you receive information from either parent in the form of alleles – again, you can think of these as versions of the same gene, containing the different forms the trait can take, such as whether eyes are blue, brown or green. Because we have two parents, we have two copies of each gene, that is, two alleles. These alleles might be the same (both parents have alleles for brown eyes), or they might be different (one has an allele for brown eyes, one has an allele for blue eyes). Where they are different, one allele will be dominant and the other recessive, and it is the former that gets expressed. Your genotype is your entire collection of alleles, and your phenotype is the body that results.

When we talk about ‘sex’, we are talking about both genotype and phenotype.

---

Footnotes

1. The discovery of DNA began in the late 1860s with the work of Swiss chemist Friedrich Miescher. The double helix structure was identified in 1953 through the work of Watson, Crick, Franklin and Wilkins.
2. The exception being those viruses which use RNA instead of DNA, assuming you regard viruses as living things, which is another debate.
3. The exception is your sex cells or gametes, which have only 23 each. I’ll discuss those in due course.

Sex and gender

In popular usage, the terms ‘sex’ and ‘gender’ are often used interchangeably, as if they meant the same thing. Sociologists, philosophers and others however mostly agree that they refer to different things. 

• Sex is biological: whether a person is male or female. It is a material reality: you have certain physical features or you don’t. A small minority of people have DSDs (sometimes called intersex), i.e. their sexual characteristics don’t fit easily into the categories of male or female.
• Gender is social: whether a person behaves in masculine or feminine ways. Gender refers to behavioural stereotypes to which people are expected to conform depending on whether they are male or female. Gender is a social construct: it has been built (and can therefore also be unbuilt). 

A whole host of other terms come piling in after. For example, neither term should be confused with sexual orientation, which is who you are sexually attracted to. The terminology and the arguments, like in any field, change over time. This can be a good or bad thing, depending on whose interests are served by the change.

I am going to delve into the topic of sex in the next few articles. Discussions of sex can get controversial and there are wildly divergent opinions. But we can only try to work out what is true, as far as we can.

For the best understanding we need to think about these categories in various ways. We must approach them scientifically, insofar as we are discussing facts about people’s actually existing bodies. We assess material reality with reference to such observable and testable features as biology, anatomy and chromosomes; and this overlaps with approaching them psychologically. We must also approach them philosophically, since we are dealing with concepts that require clarification. Finally we must consider them politically: we live in a world where resources (and therefore power) are distributed unequally, and so whether people are male or female, or masculine or feminine, has profound consequences for their lives. Politics penetrates everything. It would be naive and incorrect to suggest that the science and philosophy of sex and gender have no politics.

To begin with a caveat: describing people using labels is convenient and, to an extent, necessary. Without universal concepts, we couldn’t even hold a conversation. But concepts have their limitations: they are approximations of a world that is infinite. Every human being is particular, and like all particulars is complex and unique, living in a dynamic interaction with an infinitely complex world. Each person has their own experiences, emotions, and relationships. No actually living human being is reducible to a concept or a stereotype.